Please note that due to the lack of Unix USB API standards, the USB code in apcupsd works only on Linux and *BSD systems. In addition, at the current release (3.10.17) the USB support for *BSD systems can at best be considered BETA due to fragile kernel drivers. Drivers for other OSes can be written, but it requires someone with a knowledge of the OS and the USB to do so. (This lack of a Unix USB API interface is one of the big failings of Unix. It occurs in other areas such as the GUI. Many people tout the diversity as an advantage, but it is in fact a weakness.)
The apcupsd maintainers develop it under Fedora (Red Hat); that port is, accordingly, the most up to date and best tested. There are enough Debian Linux users that that port is also generally pretty fresh. Slackware Linux is also fully supported.
apcupsd has also been ported to FreeBSD, NetBSD, OpenBSD, HP/UX, Solaris, Alpha Unix and the Cygwin Unix emulation under Windows. It is quite likely to work on those systems, though the port may occasionally get stale and require minor tweaking. apcupsd can also work on Unix-like systems, but without USB mode. apcupsd has been ported to OS X/darwin with this limitation.
In Operating System Specifics you'll find operating-system-specific tips for building and configuring apcupsd.